This invention relates to an anaesthetic vaporiser. The vaporiser includes a pump for delivering a quantity of an anaesthetic agent accurately for administration to a patient.
It is generally necessary for anaesthetic agents to be delivered in accurately measured quantities over a period of time, for example, while the patient undergoes surgery.
As well as the requirement for accurate measurement of a dose of an anaesthetic agent, it can be desirable that a dosing pump in an anaesthetic vaporiser be capable of delivering quantities of the agent over a wide range of delivery rates. For example, the pump can be required to deliver the fluid at rates which vary by a factor of as much as 5500. For example, the rate of flow of carrier gas through the vaporiser might vary between 0.2 and 15 liters per minute, and the anaesthetic concentration might vary between 0.2 and 12% by volume.
A further problem which is encountered in the administration of anaesthetic agents is that such fluids are liable to degrade materials which are commonly used to provide fluid-tight seals in fluid handling equipment. Examples of such materials include many polymers such as that sold under the trade mark VITON Polymeric materials which are generally inert towards such fluids, such as polytetrafluoroethylene, are not generally able to provide an effective fluid-tight seal.
GB-A-2181493 discloses a pump for providing an accurately measured dose of an anaesthetic agent, which comprises a piston and a cylinder, each of which move within a housing by the action of respective eccentric drives. Movement of the piston and cylinder cause a fluid to be drawn into, and subsequently to be expelled from, a pump space. This pump suffers from the disadvantage that the pump space into which fluid is drawn is defined by components which can be moved relative to one another. Under certain operating conditions (generally involving fast operation of the pump to deliver a relatively large quantity of fluid), and when delivering certain aggressive fluids, degradation of the materials of seals provided between the moveable components can be subject to wear and then possibly to leakage of fluid from the pump space.
The present invention provides a vaporiser in which the pump which includes a reservoir for fluid defined by an expandable body.